United States Code, Title 17, is where US federal copyright laws are found.
Your tax dollars are used, in part, to maintain the US Library of Congress and one of it's departments, the US Copyright office.
www.copyright.gov
According to USC Title 17, as soon as a photo you have made is recorded on/in a tangible medium, like a digital camera's memory card, you become the copyright owner of that photo, for the rest of your life, + 70 years so you can pass that copyright ownership to your heirs.
Anyone else needs your permission to use your photos for anything, including their personal use.
You can do anything to your photos you want to.
Professional print labs may require you complete an application before they will do business with you.
There are online consumer print labs, like Mpix, that are owned and operated by professional labs. Mpix is owned and operated by Miller's Professional Imaging.
As far as doing photography as a side job, either online or in person, visit your city clerks office to fund out what local requirements for a business are.
States that have sales tax require a business collect and forward those tax dollars to them. Most states have that information online.
Once you start accepting money for your photographic sevices, homeowners insurance may no longer cover you for any injuries a client has during a shoot and they may also no longer cover your camera equipment since it is being used in a business.
As burgo pointed out, it is a very good practice to consult with an attorney, an accountant, and your insurance agent for specifiics that apply in your city, county, and state.